This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-127810, filed Apr. 27, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an electrostatic chuck for holding a workpiece such as a wafer by means of an electrostatic force in a manufacturing process for a semiconductor, for example.
A mechanical chuck, vacuum chuck, electrostatic chuck, etc. are known means for holding a workpiece in position. In a semiconductor manufacturing process, for example, an electrostatic chuck that utilizes electrostatic force may sometimes be used to hold a wafer in position. In order to maintain a desired electrostatic force, an electrical insulator such as a ceramic is used as a material for the electrostatic chuck. A sintered electrostatic chuck is described in Jpn. Pat. Appln. KOKAI Publication No. 11-260534, for example. This electrostatic chuck uses a sintered ceramic material that consists mainly of alumina (Al2O3) or aluminum nitride (AlN).
In the semiconductor manufacturing process, the electrostatic chuck may sometimes be exposed to a wide range of atmospheric temperatures, from minus tens of degrees to plus hundreds of degrees centigrade. In this case, the necessary electrostatic adsorption force cannot be obtained satisfactorily with use of pure alumina or aluminum nitride, especially in the low temperature region. Accordingly, the feasible electrical conductivity (1xc3x97108 to 1xc3x971012 xcexa9xc2x7cm) for the electrostatic chuck is obtained by doping alumina or aluminum nitride with traces of various oxides, nitrides, carbides, etc., thereby adjusting the composition of the ceramic. Besides the sintered electrostatic chuck, an electrostatic chuck (flame-coated electrostatic chuck) is known that is formed by flame-coating a metal substrate with the ceramic material.
In the semiconductor manufacturing process, the electrostatic chuck may sometimes be exposed to a corrosive environment that is based mainly on a fluoride plasma gas. In this case, the surface of the electrostatic chuck is corroded and its crystal grains detach, so that the wafer may be soiled or damaged.
Usually, an electrostatic chuck is fixed to a metal substrate by brazing, soldering, adhesive bonding, or mechanical clamping. However, there is a great difference in coefficient of thermal expansion between the metal substrate and a conventional ceramic electrostatic chuck. For example, the coefficient of thermal expansion of an alumina-based ceramic is about 8xc3x9710xe2x88x926/xc2x0C, while that of an aluminum-nitride-based ceramic is about 4xc3x9710xe2x88x926/xc2x0C. On the other hand, the coefficient of thermal expansion of a metal substrate of an aluminum alloy is as high as about 20xc3x9710xe2x88x926/xc2x0C.
Accordingly, a sudden change in temperature during the semiconductor manufacturing process causes a great difference in thermal expansion between the ceramic electrostatic chuck and the metal substrate. This difference in thermal expansion may result in deformation of the electrostatic chuck or separation or breakage of the joints between the ceramic and the metal substrate, thus constituting a hindrance to the semiconductor manufacturing process.
Accordingly, the object of the present invention is to provide an electrostatic chuck highly resistant to corrosion and subject to only a small difference in coefficient of thermal expansion between a chuck body and a metallic member.
In order to achieve the above object, according to the present invention, there is provided an electrostatic chuck comprising a chuck body having an adsorption face capable of electrostatically adsorbing a to-be-adsorbed object, such as a semiconductor wafer, and an electrode for applying a potential to the adsorption face. The chuck body is formed of a ceramic consisting mainly of magnesium oxide (MgO) and having electrical conductivity in a working temperature region. The ceramic that consists mainly of MgO is highly resistant to corrosion in a fluoride atmosphere that is used in a semiconductor manufacturing process or the like. Since this ceramic, consisting mainly of MgO, has a coefficient of thermal expansion approximate to that of metal, the difference in thermal expansion between the ceramic and a metallic member that are coupled together is small. Since the electrostatic chuck of the invention can utilize the Johnsen-Rahbek effect, moreover, it can produce a great adsorption force in a wide temperature region that is applied to a semiconductor wafer manufacturing process, for example.
According to the invention, a ceramic that consists of MgO doped with less than 1.5% of a carbide (e.g., TiC) may be used so that required electrical resistivity (1xc3x97108 to 1xc3x971012 xcexa9xc2x7cm) for the electrostatic chuck can be obtained in the working temperature region of the electrostatic chuck. Since TiC is an electrically conductive material, the electrical resistivity of the electrostatic chuck can be adjusted to the working temperature region by the addition of TiC. If 1.5% or more of TiC is added, however, the sintered density of the ceramic inevitably lowers.
According to the invention, a ceramic that consists of MgO doped with less than 5% of an oxide of a transition metal may be used so that required electrical resistivity for the electrostatic chuck can be obtained in the working temperature region of the electrostatic chuck. The oxide may be selected from a group including titanium dioxide (TiO2), zirconium dioxide (ZrO2), vanadium pentoxide (V2O5), niobium pentoxide (Nb2O5), tantalum pentoxide (Ta2O5), cobalt tetroxide (Co3O4), chromium trioxide (Cr2O3), etc.
TiO2, ZrO2, V2O5, Nb2O5, Ta2O5, Co3O4, and Cr2O3 react with MgO to form a solid solution and create electronic defects in MgO, thereby enhancing the electrical conductivity of the ceramic. Thus, the electrical resistivity of the electrostatic chuck can be adjusted to the working temperature region by adding these oxides to MgO. If 5% or more is added, however, the sintered density and corrosion resistance of the ceramic inevitably lower.
According to the present invention, moreover, MgO may be doped with less than 15% of NiO so that the required electrical resistivity for the electrostatic chuck can be obtained in the working temperature region of the electrostatic chuck. NiO also serves to form a solid solution in MgO, thereby enhancing the electrical conductivity of the ceramic. Thus, the electrical resistivity of the electrostatic chuck can be adjusted to the working temperature region by adding NiO to MgO. If 15% or more of NiO is added, however, the sintered density and corrosion resistance of the ceramic inevitably lower.
In the present invention, the chuck body may be fixed to the metallic member. According to this invention, there may be provided a high-durable electrostatic chuck comprising a metallic member and a ceramic chuck body, which cannot be deformed by a difference in thermal expansion, and in which the respective interfaces of the ceramic and the metallic member can avoid being separated or broken.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.